The present invention generally relates to a production machine for producing semiconductors, optical fiber or the like and more particularly, to a liquid replenishing apparatus for performing replenishment of liquid raw material of the semiconductors, optical fiber or the like into a raw material tank of a gas supplying apparatus arranged to supply gaseous raw material of the semiconductors, optical fiber or the like to a reaction vessel in the production machine.
Generally, optical fiber is produced by a vapor phase axial deposition (VAD) method, an outside chemical vapor deposition (outside CVD) method, an inside CVD method, etc. In the VAD method, gaseous raw material of optical fiber obtained by vaporizing liquid raw material of optical fiber in a raw material tank is burnt into glass soot and then, the glass soot is deposited on a rotating starting member so as to be formed into rodlike porous base material of glass (preform rod) such that the porous base material is subjected to spinning into optical fiber by a melting furnace. Thus, in the VAD method, it becomes necessary to provide a liquid replenishing apparatus for sequentially replenishing the raw material tank with the liquid raw material. One example of such a prior art liquid replenishing apparatus is shown in FIGS. 1 and 2. As shown in FIG. 1, liquid raw material 2 of optical fiber is stored in an airtight raw material tank 1 in which a space 3 is defined above a liquid level of the liquid raw material 2. A pipe 5 is led from the space 3 to a reaction burner (not shown) for producing a base material of glass for optical fiber by a vapor phase reaction. Thus, the liquid raw material 2 in the raw material tank 1 gradually decreases in amount upon supply of gaseous raw material of the optical fiber, i.e., saturated vapor of the liquid raw material 2 from the raw material tank 1 to the reaction burner. Accordingly, the raw material tank 1 is required to be sequentially replenished with the liquid raw material 2 as necessary by the liquid replenishing apparatus. To this end, a pipe 6 is extended, through an on-off valve 11, between a replenishment tank 7 for storing therein liquid raw material replenishment 8 for the liquid raw material 2 and the raw material tank 1. A sufficient amount of the liquid raw material replenishment 8 necessary for the raw material tank 1 is stored in the replenishment tank 7.
Meanwhile, in the replenishment tank 7, a space 9 is defined above a liquid level of the liquid raw material replenishment 8. A pipe 10 for introducing pressurized gas into the space 9 is provided so as to extend between the space 9 and a source (not shown) of the pressurized gas via an on-off valve 12 and a flow regulator 15. The pipe 10 is communicated, at a portion thereof between the flow regulator 15 and the on-off valve 12, with the pipe 6 by way of a branch pipe 6'. An on-off valve 13 is provided in the course of the branch pipe 6'. In the raw material tank 1 and the replenishment tank 7, pipes 4 and 24 are, respectively, extended, through on-off valves 16 and 36, from the spaces 3 and 9 to a gas treating apparatus for treating exhaust gas generated in the spaces 3 and 9.
Supply of the liquid raw material replenishment 8 in the known liquid replenishing apparatus shown in FIGS. 1 and 2 is performed as follows. Initially, as shown in FIG. 1, the on-off valves 13 and 36 are closed, while the on-off valves 11 and 12 are opened. Thus, since the pressurized gas, usually pressurized nitrogen gas, is introduced into the space 9 via the pipe 10, a pressure in the space 9 is raised higher than a pressure in the space 3, so that a portion of the liquid raw material replenishment 8 is supplied from the replenishment tank 7 to the raw material tank 1 via the pipe 6. A flow rate of the liquid raw material replenishment 8 delivered from the replenishment tank 7 to the raw material tank 1 is adjusted to a predetermined value by the flow regulator 15. At the time when the liquid raw material replenishment 8 has been supplied to the raw material tank 1 such that the liquid raw material 2 reaches a predetermined liquid level in the raw material tank 1, the on-off valves 11 and 12 are closed so as to stop supply of the liquid raw material replenishment 8 from the replenishment tank 7 to the raw material tank 1. At this time, the pipe 6 is filled with the liquid raw material replenishment 8. The liquid raw material replenishment 8 remaining in the pipe 6 undesirable may corrode the pipe 6 and therefore, should be discharged from the pipe 6. FIG. 2 shows a state of the known liquid replenishing apparatus in which changeover of the on-off valves 11, 12, 13 and 36 is performed from the state of FIG. 1, i.e., the on-off valves 11 and 12 are closed while the on-off valves 13 and 36 are opened. Consequently, the pressurized gas discharges the liquid raw material replenishment 8 remaining in the pipe 6 into the raw material tank 1 via the flow regulator 15 so as to empty the pipe 6 of the liquid raw material replenishment 8.
In the known liquid replenishing apparatus of FIGS. 1 and 2, a flow velocity of the pressurized gas for delivering the liquid raw material replenishment 8 to the raw material tank 1 is set at a fixed value by the flow regulator 15. As the liquid raw material replenishment 8 remaining in the replenishment tank 7 gradually reduces in amount, a volume of the space 9 in the replenishment tank 7 increases. In the case where the liquid raw material replenishment 8 is supplied to the raw material tank 1 in this state, the on-off valves 11 and 12 are opened and the pressurized gas is introduced into the space 9 via the pipe 10. However, since the flow regulator 15 is provided in the course of the pipe 10, the pressurized gas is introduced into the space 9 at a predetermined flow rate. Thus, as the volume of the space 9 becomes larger, a time period required for raising the pressure in the space 9 to a predetermined pressure increases. For example, supposing that the liquid raw material replenishment 8 is silicon tetrachloride having a specific gravity of 1.48 and the replenishment tank 7 has a capacity of 500 kg, a maximum volume Vmax of the space 9 is given by: EQU Vmax=500/1.48=338 liters
Let the flow rate of the liquid raw material replenishment 8 set by the flow regulator 15 be 10 liters/min., a time period required for raising the pressure in the space 9 to 1.5 kg/cm.sup.3 is: EQU 338.times.1.5/10=50.7 min.
Consequently, the prior art liquid replenishing apparatus has such a disadvantage that as the liquid raw material replenishment 8 remaining in the replenishment tank 7 decreases in amount, a starting point of time for starting supply of the liquid raw material replenishment 8 to the raw material tank 1 is delayed.
Furthermore, in the case where the known liquid replenishing apparatus of FIGS. 1 and 2 is used for supplying the liquid raw material replenishment 8 to a plurality of the raw material tanks 1, the known liquid replenishing apparatus of FIGS. 1 and 2 becomes complicated in structure and needs sophisticated operations by using a control device, etc. Namely, FIG. 3 shows an arrangement of the known liquid replenishing apparatus of FIGS. 1 and 2 in the case of supply of the liquid raw material replenishment 8 to a plurality of n raw material tanks 1(1), 1(2), . . . , 1(n), with the character n representing a natural number not less than 2. Supposing that a character i represents an arbitrary natural number from 1 to n, a raw material tank 1(i) is connected with the pipe 6 via an on-off valve 14(i) and an flow regulator 43(i) and is connected with a pipe 5(i). The prior art liquid replenishing apparatus of FIG. 3 further includes a variable flow control device 40, a pressure gauge 41, a control device 42 and an on-off valve 44. When some of the n raw material tanks 1(1), 1(2), . . . , 1(n) store therein insufficient amounts of the liquid raw material 2, some of the n on-off valves 14(1), 14(2), . . . , 14(n) corresponding to said some of the n raw material tanks 1(1), 1(2), . . . , 1(n) are opened so as to be ready for supply of the liquid raw material replenishment 8 to said some of the n raw material tanks 1(1), 1(2), . . . , 1(n). Signals indicative of opening and closing of said some of the n on-off valves 14(1), 14(2), . . . , 14(n) are transmitted to the control device 42. In the known liquid replenishing apparatus of FIG. 3, prior to supply to the liquid raw material replenishment 8 from the replenishment tank 7 to said some of the raw material tanks 1(1), 1(2), . . . , 1(n), the on-off valves 13 and 36 are closed, while the on-off valves 11, 12 and 44 are opened. Thus, the pressurized gas is introduced into the space 9 at a fixed flow rate through the variable flow control device 40 provided, between the on-off valves 12 and 44, in the pipe 10 so as to raise the pressure in the space 9 to a predetermined value. Consequently, as the volume of the space 9 becomes larger, a time period required for starting supply of the liquid raw material replenishment 8 from the replenishment tank 7 to said some of the raw material tanks 1(1), 1(2), . . . , 1(n) increased. An internal pressure of the replenishment tank 7 is monitored by the pressure gauge 41 and a pressure signal outputted from the pressure gauge 41 is applied to the control device 42. When the internal pressure of the replenishment tank 7 has reached a predetermined value, the liquid raw material replenishment 8 is supplied to said some of the raw material tanks 1(1), 1(2), . . . , 1(n) via the pipe 6 and the opened ones of the on-off valves 14(1), 14(2), . . . , 14(n). A flow rate of the pressurized gas into the space 9 is set by the variable flow control device 40 in accordance with the number of said some of the raw material tanks 1(1), 1(2), . . . , 1(n).
Meanwhile, since amounts of the liquid raw material replenishment 8 supplied to the raw material tanks 1(1), 1(2), . . . , 1(n) decrease as distances between the replenishment tank 7 and the raw material tanks 1(1), 1(2), . . . , 1(n) become larger, the flow regulators 43(1), 43(2), . . . , 43(n) provided respectively for the raw material tanks 1(1), 1(2), . . . , 1(n) are adjusted such that a substantially identical amount of the liquid raw material replenishment 8 is supplied to the raw material tanks 1(1), 1(2), . . . , 1(n) regardless of distances between the replenishment tank 7 and the raw material tanks 1(1), 1(2), . . . , 1(n). Thus, upon delivery of the pressurized gas into the space 9, the liquid raw material replenishment 8 is supplied to said some of the raw material tanks 1(1), 1(2), . . . , 1(n). Subsequently, the on-off valves 11 and 12 are closed, while the on-off valve 13 is opened. Thus, the pressurized gas is carried into the pipe 6 at the flow rate set by the variable flow control device 40 so as to discharge the liquid raw material replenishment 8 remaining in the pipe 6 into said some of the raw material tanks 1(1), 1(2), . . . , 1(n), so that the pipe 6 is emptied of the liquid raw material replenishment 8.
Therefore, the prior art liquid replenishing apparatus of FIG. 3 is disadvantageous in that a long time period is required for raising the pressure of the space 9 prior to start of supply of the liquid raw material replenishment 8 to the raw material tanks 1(1), 1(2), . . . , 1(n) as described above and its operations are extremely complicated.
Furthermore, the prior art liquid replenishing apparatus of FIG. 3 has such an inconvenience that the variable flow control device 40, the pressure gauge 41, the control device 42, etc. are required to be provided additionally, thereby resulting in rise of its production cost.